In electrostatographic apparatus, such as electrophotographic copiers and printers, an electrostatic image pattern is formed by uniformly charging and imagewise exposing a photoconductor member. The latent electrostatic image is then developed by supplying charged toner particles that can be attracted to the photoconductor member to form a corresponding, or opposite sense, toner image. The toner image is then transferred to a copy sheet, or to an intermediate transfer member that subsequently re-transfers it to a copy sheet. The transfer from the photoconductor is most often effected by use of electrical potentials that cause the charged toner particles to move away from the photoconductor. The re-transfers from the intermediate transfer member can be effected by electrical potentials, by heating the toner particles to a tacky state and pressing them into binding contact with the copy sheet fibers or by a combination of those processes.
In each of the transfer processes, it is very desirable that substantially all of the toner comprising the image pattern be transferred. Complete transfer enhances the apparatus performance from two general view points, viz: (I) no residual toner remains on the photoconductor or intermediate transfer member to impede or degrade subsequent image operations and (ii) the complete toner pattern ends up at the desired site, so that copies with proper density and/or color balance are attained. However, complete transfers are difficult to achieve, particularly for minimum density image regions and with regard to very small particle toners such as used in high resolution multicolor images.
To improve completeness of transfer, prior art apparatus have provided special release surfaces on intermediate transfer members and applied release liquids to those members, using wicks or brushes (see, e.g., U.S. Pat. No. 5,434,657). On photoconductor members, clear toner with desirable release characteristics has been applied, e.g. with magnetic brush applicators, as an underlayer for the toner image particles. Such prior art techniques are useful but have disadvantages. For example, the wick and brush release liquid applicators are not imagewise selective and can be contaminated and/or wear out the surfaces that they contact. Using underlayers of clear toner particles can increase the final image thicknesses considerably in instances where 3 or 4 different image layers, each with an accompanying underlayer, are superimposed.